The present invention relates generally to the field of gas blending apparatus. More particularly, this invention relates to a gas blending system in which a proportional blending valve is set and controlled electronically. The invention has specific applications in the medical field, especially in ventilators and anesthesia apparatus.
The precise blending of two or more gases is important in many applications, particularly in the medical field, where a precisely regulated mixture of gases must be administered to a patient. For example, in a ventilator or "respirator", it is usually desired to deliver a blend of oxygen-enriched air (i.e., more than 21 percent oxygen) to the patient being ventilated. Also, patients undergoing surgery under general anesthesia must be administered a precisely regulated mixture of anesthetic gas and respiratory gas (air, oxygen, or oxygen-enriched air). In either case, the proportional blend of gases must be set and maintained with accuracy.
A typical prior art blending system for a medical ventilator is disclosed in U.S. Pat. No. 4,072,148 to Munson, et al. In this prior art system, pressure-regulated air and oxygen are separately delivered to a mixing valve which is manually adjusted to achieve the desired proportional blend of the two gases. The valve itself has one flow path from an air inlet to a blended gas outlet, and another flow path from an oxygen inlet to the outlet. A valving element, the position of which is manually adjusted, opens one of the flow paths as it proportionately closes the other, thereby allowing the selection of a broad range of proportional air/oxygen blends.
While this system has achieved very satisfactory results, it does have some limitations. For example, the accuracy of the proportions of the blended gases, in terms of the variance of the actual proportions from their nominal values as set by the operator through the manual control, depends upon the accuracy of the calibration of the control dial or knob.
Another limitation stems from the need, in a medical ventilator, to accommodate a wide range of gas flow rates. Specifically, in a blending system using a mixing valve of fixed total flow area, the mixing accuracy depends on maintaining a balance in the regulated pressures of the gases entering the mixing valve. An imbalance in these pressures adversely affects the accuracy of the blending system. The error introduced by such imbalances is usually not significant at high flow rates, when the pressure drop across the mixing valve is considerably greater than the expected range of imbalance in the regulated gas pressures. At lower gas flow rates, however, the pressure drop across the valve decreases, and the effect of a regulated pressure imbalance becomes correspondingly more significant. On the other hand, if the flow rate is too high, the pressure drop across the valve is excessive, leaving inadequate gas pressure to overcome the pneumatic resistance of the ventilator so that gas can reach the patient.
Consequently, a gas blending valve with a fixed total flow orifice area is operative, with a suitable degree of accuracy, only within a relatively narrow range of flow rates. It has been found that the range of peak flow rates in an adult volume ventilator should be, advantageously, from about 5 liters per minute (1 pm) to about 150 l pm, and perhaps higher. This range is too broad to be handled by a "single stage" mixing valve (that is, one having a single total flow orifice of fixed area, divided proportionately by the valving element). The prior art system described above approaches this problem by using a "multistage" mixing valve, in which the valve is divided into two or more valve "modules" characterized by progressively increasing volumetric flow capacities. As the flow rate increases, the higher flow capacity modules are sequentially opened, and as the flow rate decreases, they are sequentially closed. This structure allows the valve to operate throughout a very broad range of flow rates without degrading absolute mixing accuracy, as might occur by low flow rates through the higher capacity valve modules.
While multistage mixing valves can effectively broaden the range of flow rates accommodated by a gas blending system, they do so at the expense of increased mechanical complexity. In addition, even multistage valves can be subject to inaccuracies as a result of pressure transients generated upstream of the mixing valve by changes in downstream demand. In a medical ventilator, for example, rapid changes in delivered rates of gas flow due to instantaneous changes in the patient's demand for gas can be transmitted to the pressure regulation system too rapidly for compensation by the gas pressure regulators. The result is a pressure transient which causes a temporary imbalance in the regulated gas pressures, with a resultant deviation from the desired gas mixture.
Another approach to achieving higher accuracy in a gas blending system is exemplified by U.S. Pat. No. 4,345,612 to Koni, et al. This system employs an electrically-controlled "throttle" valve downstream from each gas regulator. Thus, both the pressure and flow rate of each gas to be blended are separately controlled before the gases are mixed in a manifold downstream from the outlets of the throttle valves. The flow rate through each throttle valve is measured by a flow rate sensor, which provides a feedback signal to the electronic circuitry which actuates the throttle valves.
The flow rate feedback feature of the Koni, et al. system provides an added degree of control, while the use of a separate electronically-controlled throttle valve for each gas provides an alternative approach (to multistage mixing valves) to broadening the useful flow rate range of the system. Nevertheless, this system is mechanically complex, as exemplified by its need for a separate throttle valve and flow rate sensor for each gas to be blended.
It can thus be appreciated that it would be highly desirable to provide a gas blending system which accommodates a wide range of flow rates without undue mechanical complexity, but which achieves, at the same time, accurate control of the gas blend. It would also be advantageous to provide in such a system the ability to mitigate the deleterious effects of downstream dynamic flow conditions, such as can be produced, in a ventilator, for example, by changes in patient demand.